<?=form_open($this->uri->uri_string());?>
<label>Options: </label>
<select name="option">
    <option value="default" <?if($this->uri->segment(2) == 'index') {?>selected<?}?>>Default</option>
    <option value="by_referer" <?if($this->uri->segment(2) == 'by_referer') {?>selected<?}?>>By Referer</option>
</select>
</option>
<input type="submit" name="btn_submit_option" value="Submit">
</form>